Given below are two statements : one is labelled as Assertion A and the other is labelled as Reason R :
Assertion A : Acetal / Ketal is stable in basic medium.
Reason R : The high leaving tendency of alkoxide ion gives the stability to acetal / ketal in basic medium.
In the light of the above statements, choose the correct answer from the options given below :
Assertion A : Acetal / Ketal is stable in basic medium.
Reason R : The high leaving tendency of alkoxide ion gives the stability to acetal / ketal in basic medium.
In the light of the above statements, choose the correct answer from the options given below :
- A.Both A and R are true but R is NOT the correct explanation of A
- B.Both A and R are true and R is the correct explanation of A
- C.A is false but R is true
- D.A is true but R is false
Show answer and solution
Correct option: D
Solution
Acetals/ketals are stable in basic medium — this is true (A is correct). They are formed by reaction of aldehydes/ketones with alcohols in acid catalysis, and base cannot catalyze their hydrolysis because the mechanism requires protonation of the oxygen to generate a good leaving group (oxocarbenium ion). In basic medium, the alkoxide ion (RO⁻) is actually a very POOR leaving group (strong base/poor leaving group tendency), so the C–O bond cannot be cleaved by hydroxide attack. Therefore, R is false — it is precisely the LOW leaving tendency of alkoxide that makes acetals/ketals stable in base, not the high leaving tendency.
[JEE Main 2023 · 25 Jan · Shift 1]